Our invention relates to a radio transceiver or communication radio transmitter and receiver, and particularly to a radio transceiver that operates on a plurality of spaced radio channels, such as in a radio telephone system.
Radio telephone systems have improved to the extent that the Federal Communications Commission has approved a nationwide cellular radio telephone system. In the cellular system, 666 channels with 30 Kilohertz spacing have been allocated. These channels lie in the band of 825 to 845 Megahertz for mobile transmitting and base station receiving; and in the band of 870 to 890 Megahertz for mobile receiving and base station transmitting. The system provides full duplex operation, so that for a given communication, two companion channels are used, these channels being separated by 45 Megahertz. Thus, communication on designated channel 1, for example, would use 825.030 Megahertz for mobile transmitting and base station receiving, and 870.030 Megahertz for mobile receiving and base station transmitting.
The electronic equipment needed in such a cellular system is obviously relatively complex. But at the same time, such equipment must meet the relatively high Federal Communications Commission standards on low noise and high frequency stability, yet must be relatively inexpensive so as to be available to as many users as possible.
A cellular transceiver for which our invention is intended uses a single, relatively stable crystal oscillator that serves as a reference for a receiving frequency synthesizer, and for a transmitting frequency synthesizer that can be modulated. In such a transceiver, the receiving frequency synthesizer output is mixed with the received frequency to produce a receiving intermediate frequency, and the receiving frequency synthesizer output is also mixed with the transmitting frequency synthesizer output to produce the transmitted frequency. Because of coupling, such as through a duplexer or adjacent antennas, between the transmitted output and received input, the high power transmitted signal may be mixed with the received signal to produce another intermediate frequency that is very close to the receiving intermediate frequency. If the crystal oscillator drifts, as it well may with changes in temperature, these two intermediate frequencies produce an audible beat note that is undesirable and may interfere with reception.